More concrete and column research

Some months ago, I published a post about some research I was undertaking as part of my honours project at the University of Adelaide about the use of high strength concretes and a column design – which consumed a lot of my time and kept me from updating this blog for several months late last year. This work was completed in October last year.

For reasons of confidentiality, I can not disclose detailed information about the design specifications of the columns or the testing apparatus we used. However, I can explain the basics of the testing program that was undertaken without going into too much technical detail.

The aim of our research was to explore the feasibility of creating a high-strength, high-performance column known as a Double-Skin Tubular Column (DSTC). These columns consisted of several components: an internal steel tube, an outer tube made from layers of fibre reinforced polymers (FRP), and the space between the two tubes filled with high strength concrete.

Plan view of the column from above. There is a hollow steel tube surrounded by concrete which in turn is confined by an outer FRP tube.

Our testing program was undertaken in two stages. The first involved creating concrete mixes with differing strengths – compressive strengths of 90 Mpa and 110 Mpa were used – and testing them 28 days after pouring the batches to check that the strengths being achieved were suitable to be used in the test columns. Photos and footage of this testing can be seen in the previous post on this research.

The second phase to our testing program involved the task of constructing 1.6 metre tall columns with supporting footings. Each of these columns was different from each other to examine the effect of altering the basic design by changing the concrete strengths, the thickness of the outer FRP tube (or the confinement as it is known by structural engineers), FRP type as well as the effect of different loads on the column. The following photos show the columns in their construction phase.

Steel bar cages were prepared for the foundation, while the steel tube was welded for anchorage.

The outer FRP tubes being cast layer by layer on a special rotating apparatus.

Columns awaiting concrete to be cast in-situ between the two tubes after their foundations were poured.

To test these columns, a test rig was constructed in the labs and hydraulic jacks were used to apply a constant vertical load as well as a varying horizontal load on the column. In effect, the columns were pushed back and forth to simulate seismic loading and tested to failure. Information about the column performance was obtained using strain gauges attached to various points on the column. The following photos and video shows the columns during testing. Note that the video is sped up to about 20 times normal speed.

Column placed in the test rig awaits its destiny.

A column in testing, being pushed far off centre.

An image of the FRP tube at column failure.

Damage to the concrete after stripping away the outer FRP tube.

In a nutshell, our research found that the columns performed very well with high bending capacities and high levels of lateral displacement compared to conventional columns.